Open the GarageBand app on your iPhone
Tap the plus button to create a new project
Choose an audio recorder or instrument
Tap the Tracks view button
Tap the Loop Browser icon
Import the song or audio file you want to use
Drag the audio into the timeline
Trim the audio to 30 seconds or less
Tap the down arrow and select My Songs
Press and hold the project, then tap Share
Tap Ringtone
Enter a ringtone name
Tap Export
Choose Use sound as
Select Standard Ringtone, Text Tone, or assign it to a contact
Open Settings
Tap Sounds & Haptics
Tap Ringtone
Select your new ringtone